Yay! All fixed....and I didn't even have to get down on the floor. Managed to reach the fuses sitting on a stool. Pulled out the two 40-amp orange fuses and both were blown. Quick trip to Advance Auto & $4 later, new fuses are in place & everything is working fine.
Thanks again for the photos, Fonzie! That saved me pulling out all the fuses & checking them.
In one of the fuses, the filament was clearly burnt in two. In the other, it was just barely apart. Wondering if perhaps that's why I was able to open the slide-out when it first happened? Maybe the filament halves were still making some contact but then heat/vibration/something finally separated them completely. Whatever the reason, it's all better now.
Thanks for all the replies. I don't post here a lot but it's so great to know I can get quick & helpful advice when I need it!